1. Field of Invention
This invention relates to devices for measuring electrostatic fields. This invention also relates to methods of making the devices.
2. Description of Related Art
Electrostatic voltmeter modulators (ESV) are used to measure electrostatic fields associated with electrostatic charges on surfaces. Electrostatic voltmeter modulators provide an output signal that can be used to control an electrostatic process as a function of the measured electrostatic potential. Electrostatic voltmeter modulators enable the adjustment of apparatus characteristics during operation based on the sensed electrostatic field.
In electrostatic voltmeter modulators, the distance or electronic coupling between two surfaces that are capacitively coupled to each other can be mechanically changed relative to time, or modulated. This modulation changes the capacitance of the electrostatic voltmeter modulator with respect to time. In electrostatic voltmeter modulators, a sensing electrode is modulated with respect to the electrostatic field being measured. Different methods of achieving the required modulation of the electrode are known. In some methods, the electrode remains stationary and a vibrating element, or vane, is moved to modulate the field reaching the electrode.
Another method of modulating the sensing electrode involves moving the electrode, which is vibrated relative to the surface being measured. One version of this method moves the electrode in a direction perpendicular to the surface being measured, thereby directly varying the capacitance between the electrode and the surface by changing the distance between the electrode and surface.
Another version utilizes an electromechanical system, such as a xe2x80x9cchopper,xe2x80x9d which mechanically interrupts the coupling between the two surfaces.
As described above, electrostatic voltmeter modulators are used to measure electrostatic fields. During operation of electrostatic voltmeter modulators to measure electrostatic fields, it is important that electrostatic charge does not accumulate on the electrostatic voltmeters themselves. Such charge build-up affects measurement accuracy because it produces a corresponding electrostatic field that is sensed by the electrostatic voltmeter modulator in addition to the electrostatic field corresponding to electrostatic charge present on the test surface. As a result of charge build-up on electrostatic voltmeter modulators and the added associated electrostatic field produced by the charge build-up, zero shift (which is also known as drift) of the electrostatic voltmeter modulator can occur. Consequently, the electrostatic voltmeter modulator is unable to provide stable and accurate measurements of the actual voltage on the test surface due to the zero shift caused by the charge build-up.
One exemplary use of electrostatic voltmeter modulators is to measure electrostatic charge on photoreceptor surfaces in electrostatic imaging apparatus. Electrostatic reproduction involves charging a photoconductive member, or photoreceptor, and imagewise discharging it, or imagewise exposing it, based on light reflected from an original image being reproduced. The result is an electrostatically-formed latent image on the photoconductive member. The latent image is developed by bringing a charged developer material into contact with the photoconductive member.
To achieve satisfactory imaging in electrostatic imaging apparatus, electrostatic charge on photoconductive members needs to be controlled. This electrostatic charge, and also the electrostatic field associated with the electrostatic charge, changes normally during operation of electrostatic imaging apparatus due to changes in various conditions, including, for example, temperature and pressure. Accordingly, to maintain desired electrostatic charge levels on photoconductive member surfaces, the electrostatic charge level needs to be accurately monitored and adjusted as needed during operation of electrostatic imaging apparatus to compensate for such condition changes. Otherwise, it is at least extremely difficult to accurately control electrostatic charge levels on photoconductive surfaces in response to changing conditions that affect electrostatic charge. Consequently, desired image quality may not be achieved.
U.S. Pat. No. 5,489,850, which is incorporated herein by reference in its entirety, discloses electrostatic voltmeter modulators including antistatic layers and antistatic material coatings. These antistatic layers and material coatings have improved device performance; however, they have not proven to be totally satisfactory to address the problem of charge build-up. Namely, the antistatic layers and material coatings have resistivity properties that are not sufficiently controllable for use in electrostatic voltmeter modulators according to the present invention, to sufficiently reduce charge build-up on electrostatic voltmeter modulators in a consistent manner. Particularly, antistatic materials disclosed in the U.S. Pat. No. 5,489,850 have resistivity values that can vary significantly and do not fall consistently within a desired range for use in the present invention. Namely, the resistivity values of these antistatic materials are not sufficiently consistent and can be either too high or too low. If the resistivity of these antistatic materials is too high, antistatic charge is not dissipated sufficiently quickly from electrostatic voltmeter modulators to substantially prevent charge buildup. The resitivity of these antistatic materials can also be too low and degrade sensitivity of electrostatic voltmeter modulators. In either case, measurement errors can occur in electrostatic voltmeter modulators.
Also, antistatic coating materials disclosed in U.S. Pat. No. 5,489,850 are adhesive materials. Although these adhesive materials can be applied on surfaces by some techniques, they have properties making them unsuitable for applying in a controlled manner by certain other coating processes.
This invention provides electrostatic voltmeter modulators that overcome above-described disadvantages of known devices and enable the measurement of electrostatic fields from charged surfaces with improved accuracy. The electrostatic voltmeter modulators have reduced zero shift, or drift, characteristics, as compared to known devices.
This invention separately provides imaging apparatus that include one or more electrostatic voltmeter modulators that can measure electrostatic fields from charged surfaces with improved accuracy.
Exemplary embodiments of the electrostatic voltmeter modulators for measuring an electrostatic field between the electrostatic voltmeter modulator and a charged surface according to this invention comprise a shield, a sensing electrode and an insulative layer between the shield and the sensing electrode. An antistatic coating is formed on at least the insulative layer.
The antistatic coatings of this invention have advantageous properties. Namely, the antistatic coatings have conductivity properties that provide excellent charge dissipation and substantially eliminate the accumulation of electrostatic charge on the insulative layer. Consequently, the electrical field measured by the electrostatic voltmeter modulator corresponds substantially to an electrical field associated with electrostatic charges on the surface being measured and not to any charge accumulated on the insulative layer.
In addition, the antistatic coatings of this invention can be applied by coating techniques that are not completely suitable for applying antistatic materials that are adhesive materials.